configs/qemu_sh4*: switch to initramfs

Qemu 9.0.0/9.0.1 has introduced a Bug in sh4/sh4eb emulation so
that the system doesn't boot anymore.
The issue is already reported by Guenter Roeck:
https://lists.nongnu.org/archive/html/qemu-devel/2024-05/msg00606.html

Switch to initramfs for sh4/sh4eb emulation.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/qemu/sh4-r2d/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
-  qemu-system-sh4 -M r2d -kernel output/images/zImage -drive file=output/images/rootfs.ext2,if=ide,format=raw -append "rootwait root=/dev/sda console=ttySC1,115200 noiotrap" -serial null -serial stdio -net nic,model=rtl8139 -net user # qemu_sh4_r2d_defconfig
+  qemu-system-sh4 -M r2d -kernel output/images/zImage -append "console=ttySC1,115200 noiotrap" -serial null -serial stdio -net nic,model=rtl8139 -net user # qemu_sh4_r2d_defconfig
 
 The login prompt will appear in the terminal that started Qemu.
 The graphical window is the framebuffer.

board/qemu/sh4eb-r2d/readme.txt

@@ -1,6 +1,6 @@
 Run the emulation with:
 
-  qemu-system-sh4eb -M r2d -kernel output/images/zImage -drive file=output/images/rootfs.ext2,if=ide,format=raw -append "rootwait root=/dev/sda console=ttySC1,115200 noiotrap" -serial null -serial stdio -net nic,model=rtl8139 -net user # qemu_sh4eb_r2d_defconfig
+  qemu-system-sh4eb -M r2d -kernel output/images/zImage -append "console=ttySC1,115200 noiotrap" -serial null -serial stdio -net nic,model=rtl8139 -net user # qemu_sh4eb_r2d_defconfig
 
 The login prompt will appear in the terminal that started Qemu.
 The graphical window is the framebuffer.

configs/qemu_sh4_r2d_defconfig

@@ -7,7 +7,7 @@ BR2_SYSTEM_DHCP="eth0"
 BR2_TARGET_GENERIC_GETTY_PORT="ttySC1"
 
 # Filesystem
-BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_INITRAMFS=y
 # BR2_TARGET_ROOTFS_TAR is not set
 
 # Image

configs/qemu_sh4eb_r2d_defconfig

@@ -6,7 +6,7 @@ BR2_sh4eb=y
 BR2_TARGET_GENERIC_GETTY_PORT="ttySC1"
 
 # Filesystem
-BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_INITRAMFS=y
 # BR2_TARGET_ROOTFS_TAR is not set
 
 # Image